Hydrologic properties may be estimated in the laboratory on field cores or from special tests conducted in the field. By determining soil water content (θ) and water potential (Ψm), various properties of a sample including soil water flux (q), hydraulic gradient (I), hydraulic conductivity (K) and the soil water retention curve [soil water potential (Ψm) versus water content (θ)] may be estimated. This data is necessary for site characterization, including determination of recharge at waste disposal, agricultural and other sites, as well as for ground water protection and conservation.
Laboratory testing requires removal of a sample from its native environment, causing disturbance to the sampling location. The sample must be transported to the laboratory for testing by a skilled technician. Labor and expense is involved with securing an undisturbed transport of the sample. The laboratory tests normally require several weeks to perform. The data obtained may be incomplete, particularly in the wet range, as the pristine condition of the sample is compromised. Thus, laboratory testing is not optimum as it is time consuming, expensive, and incomplete results may be obtained.
Field testing also requires several weeks to months to perform. While the sample may be analyzed in its native environment, other factors, including weather, temperature and wildlife may affect the testing. Similar to laboratory testing, field testing is also expensive in time, money and human resources to perform.
Estimation of unsaturated hydraulic properties under field conditions is frequently accomplished using the instantaneous profile method (IPM) (Davidson, et al., 1969). According to the IPM, when water movement takes place in a one-dimensional system, the soil moisture equation can be written as (Davidson, et al., 1969):                                           ⅆ                          W              ⁡                              (                                  z                  ,                  t                                )                                                          ⅆ            t                          =                              q            ⁡                          (                              0                ,                t                            )                                -                      q            ⁡                          (                              z                ,                t                            )                                                          [        1        ]            Where q(z,t) is the Darcian flux (LT−1) at depth z and time t, W is total water stored above z or                               W          ⁡                      (                          z              ,              t                        )                          =                              ∫            0            z                    ⁢                                    θ              ⁡                              (                                  z                  ,                  t                                )                                      dt                                              [        2        ]            When the soil column is undergoing drainage only q(0,t)=0, Equation [1] becomes:                                           ⅆ                          W              ⁡                              (                                  z                  ,                  t                                )                                                          ⅆ            t                          =                  -                      q            ⁡                          (                              z                ,                t                            )                                                          [        3        ]            Darcian flux is given by:                               q          ⁡                      (                          z              ,              t                        )                          =                              -            K                    ⁢                                          ⁢                                    ∂              H                                      ∂              Z                                                          [        4        ]            Where K is hydraulic conductivity (LT−1), H is total water potential (L) and q is approximated as:                               q          ⁡                      (                          z              ,              t                        )                          ·                  -                                                    d                ⁢                                                                  ⁢                                  W                  ⁡                                      (                                          z                      ,                      t                                        )                                                              -                              W                ⁡                                  (                                      z                    ,                                          t                      -                                              Δ                        ⁢                                                                                                  ⁢                        t                                                                              )                                                                    Δ              ⁢                                                          ⁢              t                                                          [        5        ]            When the gradient term is approximated as a difference equation, it results in the IPM for obtaining hydraulic conductivity as a function of either water content or water potential:                               K          ·                      q            ⁡                          (                              z                ,                t                            )                                      ⁢                              Δ            z                                Δ            ⁢                                                  ⁢            H                                              [        6        ]            
Implementing the IPM in the field requires wetting the soil to field saturation, covering the experiment to prevent evaporation (i.e., set q(0,t)=0) and allowing the soil to drain freely. Since the IPM requires infiltration of 1 to 10 m3 of water, as well as instrument installations at multiple depths, the method is not being used at many hazardous waste sites. The entire IPM can take several weeks in the field (Davidson, et al., 1969; Sisson, et al., 1980). The apparatus and methods disclosed herein allow rapid determination of the soil hydraulic properties while using the IPM model.